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If 56-bit DES is considered too insecure, one can simply run the 56-bit algorithm multiple times, taking the 64bit output from one iteration of DES as the input to the next DES iteration, using a different encryption key each time For example, so-called triple-DES (3DES), is a proposed US government standard [NIST 1999b] and has been proposed as the encryption standard for the Point-to-Point protocol [RFC 2420], PPP, for the data link layer (see section 57) A detailed discussion of key lengths and the estimated time and budget needed to crack DES can be found in [Blaze 1996] We should also note that our description above has only considered the encryption of a 64-bit quantity When longer messages are encrypted, which is typically the case, DES is often used with a technique known as cipher-block chaining, in which the encrypted version of the jth 64-bit quantity of data is XOR'ed with the (j +1)st unit of data before the (j+1)st unit of data is encrypted
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For more than 2000 years (since the time of the Caesar cipher and up to the 1970's), encrypted communication required that the two communicating parties share a common secret - the symmetric key used for encryption and decryption One difficulty with this approach is that the two parties must somehow agree on the shared key; but to do so requires (presumably secure) communication! Perhaps the parties could first meet and agree on the key in person (eg, two of Caesar's centurions might meet at the Roman baths) and thereafter communicate with encryption In a networked world, however, communicating parties may never meet and may never converse except over the network Is it possible for two parties to communicate with encryption without having a shared secret key that is known in advance In 1976, Diffie and Hellman [Diffie 1976] demonstrated an algorithm (known now as Diffie-Hellman Key Exchange) to do just that - a radically different and marvelously elegant approach towards secure communication that has led to the development of today's public key cryptography systems We will see shortly that public key cryptography systems also have several wonderful properties that make them useful not only for encryption, but for authentication and digital signatures as well The ideas begun with [Diffie 1976] have evolved, with a significant milestone being [RSA 1978], into the public key systems in use today
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Figure 72-5: Public key cryptography The use of public key cryptography is quite simple Suppose Alice wants to communicate with Bob As shown in Figure 72-5, rather than Bob and Alice sharing a single secret key (as in the case of symmetric key systems), Bob (the recipient of Alice's messages) instead has two keys - a public key that is available to everyone in the world (including Trudy the intruder!) and a private key that is known only to Bob In order to communicate with Bob, Alice first fetches Bob's public key Alice then encrypts her message to Bob using Bob's public key and a known (eg, standardized) encryption algorithm Bob receives Alice's encrypted message and uses his private key and a known (eg, standardized) decryption algorithm to decrypt Alice's message In this manner, Alice can send a secret message to Bob without either of them having to have to distribute any secret keys! Using the notation of Figure 72-5, for any message m, dB(eB(m)) = m, ie, applying Bob's public key then Bob's private key to the message m gives back m We will see shortly that we can interchange the public key and private key encryption and get the same result, that is, eB(dB(m)) = dB(eB(m)) = m The use of public key cryptography is thus conceptually simple But two immediate worries may spring to mind A first concern is that although an intruder intercepting Alice's encrypted message will only see gibberish, the intruder knows both the key (Bob's public key, which is available for all the world to see) and the algorithm that Alice used for encryption Trudy can thus mount a chosen plaintext attack, using the known standardized encryption algorithm and Bob's publicly available encryption key to encode any message she chooses! Trudy might well try, for example, to encode messages, or parts of messages, that she suspects that Alice might send Clearly, if public key cryptography is to work, key selection and encryption/decryption must be done in such a way that it is impossible (or at least so hard to be impossible for all practical purposes) for an intruder to either determine Bob's private key or somehow otherwise decrypt or guess Alice's message to Bob A second concern is that since Bob's encryption key is public, anyone can send an encrypted message to Bob, including Alice or someone claiming to be Alice In the case of a single shared secret key, the fact that the sender knows the secret key implicitly identifies the sender to the receiver In the case of public key cryptography, however, this is no longer the case since anyone can send an encrypted message to Bob using
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